DESCRIPTION: (Applicant's abstract) Multiple lines of evidence suggest that serotonin1A (5HT1A) receptor function is abnormal in major depressive disorder (MDD) and that somatic antidepressant therapies effect changes in 5HT1A receptor function that are relevant to treatment efficacy. The data supporting these hypotheses have been obtained by assessing neuroendocrine and temperature responses to 5HT1A agonists in MDD subjects, measuring 5HT1A receptor binding in brain tissue acquired post mortem from small samples of MDD subjects, and examining changes in 5HT1A receptor binding in rats following antidepressant drug (AD) administration. However, there has been no direct demonstration of a central 5HT1A receptor abnormality or an effect of antidepressant treatment on 5HT1A receptor pharmacology in living depressed subjects. The development of a highly selective 5-HT1A receptor radioligand for positron emission tomography (PET) imaging, [carbonyl-11C]- WAY-100635, has recently made direct, noninvasive exploration of the central 5HT1A receptor binding potential (BP) possible in MDD. To advance knowledge regarding the neurobiology of MDD and the mechanisms of AD treatment, PET and [carbonyl-11C]WAY-100635 will be used to compare the 5HT1A receptor BP between MDD and healthy control subjects and between the pre-and post-treatment conditions in the MDD subjects. Images of 18F-fluorodeoxyglucose (FDG) uptake will be acquired in the same scan session to evaluate the relationship between regional 5HT1A receptor BP and the glucose metabolic abnormalities previously identified in MDD. Endocrine assessments of cortisol secret-ion are also examined prior to imaging to determine whether down-regulation of hippocampal 5HT1A receptors occurs in response to the hypercortisolism associated with MDD as it does in experimental animals during stress and glucocorticoid administration. The imaging and endocrine measures are repeated following an 8 week interval during which the depressives are treated with the antidepressant drug sertraline. Pilot imaging data suggest that the differences in the 5HT1A receptor BP between unmedicated depressives and controls are robust in the mesiotemporal cortex and the ventrolateral and ventromedial prefrontal cortex (PFC). If a link between reduced 5HT1A receptor BP in the mesiotemporal cortex (which includes the hippocampus) and abnormal cortisol secretion is established in MDD, then normalization of this abnormality during treatment may have prognostic implications. In the ventrolateral and ventromedial PFC, the magnitude of the abnormal reductions of 5HT1A receptor BP were proportionately larger than the abnormalities of metabolism and grey matter volume previously shown in these areas in MDD, suggesting the 5HT1A receptor imaging measures may provide sensitive markers of pathology that can guide future post mortem histological and histochemical studies of MDD.